The effect of rotational isomerism on the values of first hyperpolarizability is studied by the example of two chromophores 7-DMA-VQV-TCP and 3-DMA-VQ(on)V-TCP with quinoxaline and quinoxalinone moieties in the pi-electron bridge. At the isomers formation the rotations about three single bonds are considered, thus providing eight possible conformers for each chromophore. The analysis of the results of DFT calculations demonstrates the presence of one lowest-energy conformer characterized by dominating probability for each chromophore. The calculations show that for both chromophores all rotational isomers are characterized by the first hyperpolarizability values rather close (within 8% and 11%) to statistically averaged ones. (C) 2017 Elsevier B.V. All rights reserved.